Twitter said it identified and fixed the cross site scripting flaw that led to meltdown on Tuesday a month ago, only to undo this fix with a later web site update.
The bug was mostly used for mischief but there were incidents of porn and shock site redirects. A worm, without a malicious payload, took advantage of the vulnerability to cause people to retweet the original Tweet after they rolled their mouse over a link, creating hundreds of thousands of spam message in the process.
Only surfers using Twitter.com were exposed to the vulnerability. Third party clients were unaffected.
A Japanese developer called Masato Kinugawa is credited with discovering the flaw last month, and used it to post multi-coloured rainbow tweets. Scandinavian developer Magnus Holm developed at least one of the "worms" that took advantage of the vulnerability, The Guardian reports. Holm created the worm to test what was possible little expecting that his creation would spread so quickly.
Up to 500,000 people (100 per second) may have fallen victim to the cross site scripting attack, according to an analysis by Kaspersky Labs.
Twitter said that while the attack created a huge amount of spam and confusion no greater security threat was posed and users of compromised accounts need not change their passwords.
"There is no need to change passwords because user account information was not compromised through this exploit," Twitter has reassured users in a blog post post-mortem of the incident. ®